Uncontrolled cell proliferation is the hallmark of cancer. Cancerous tumor cells typically have some form of damage to the genes that directly or indirectly regulate the cell-division cycle.
The progression of cells through the various phases of the cell cycle is regulated by a series of multienzyme complexes consisting of a regulatory protein, a cyclin, and a kinase. These kinases are called cyclin-dependent kinases (Cdks). The Cdks are expressed throughout the cell cycle, while the levels of the cyclins vary depending on the stage of the cell cycle.
The transition from G1 phase into S phase is regulated by the complex of Cdk4 with cyclin D. This complex phosphorylates the tumor suppressor protein Retinoblastoma (pRb), releasing the transcription factor E2F and allowing the expression of genes required in S phase (Nevins, J. R. Science 1992, 258, 424–429). Blocking the activity of the Cdk4/cyclin D complex arrests the cell cycle in G1 phase. For example, the proteins of the INK4 family, including p16INK4a, which block the kinase activity of the Cdk4/cyclin D complex, cause arrest in G1 (Sherr, C. J. Science 1996, 274, 1672–1677).
Recent experiments show that the complex of Cdk4 with cyclin D3 also plays a role in cell cycle progression through G2 phase. Inhibition of this complex, either by p16 or using a dominant negative Cdk4, results in arrest in G2 phase in cells that do not express pRb (Gabrielli B. G. et al. J. Biol. Chem. 1999, 274, 13961–13969).
Numerous defects in the pRb pathway have been shown to be involved in various cancers. For example, overexpression of Cdk4 has been observed in cases of hereditary melanoma (Webster, K. R. Exp. Opin. Invest. Drugs 1998, 7, 865–887); cyclin D is overexpressed in many human cancers (Sherr, C. J. Science 1996, 274, 1672–1677); p16 is mutated or deleted in many tumors (Webster, K. R. Exp. Opin. Invest. Drugs 1998, 7, 865–887); and pRb function is lost through mutation or deletion in many human cancers (Weinberg, R. A. Cell 1995, 81, 323–330). Defects in this pathway have also been shown to have an effect on prognosis. For example, loss of p16 is correlated with poor prognosis in non-small-cell lung carcinoma (NSCLC) and malignant melanoma (Tsihlias, J. et al. Annu. Rev. Med. 1999, 50, 401–423).
Because of the involvement of the Cdk4/cyclin D/pRb pathway in human cancer through its role in regulating progression of the cell cycle from G1 to S phase, and the potential therapeutic benefit from modulating this pathway, there has been considerable interest in agents that inhibit or promote elements of this pathway. For example, effects on cancer cells have been shown using antibodies, antisense oligonucleotides and overexpression or addition of proteins involved in the pathway. See, e.g., Lukas, J. et al. Nature 1995, 79, 573–582; Nevins, J. R. Science 1992, 258, 424–429; Lim, I. K. et al. Molecular Carcinogenesis 1998, 23, 25–35; Tam, S. W. et al. Oncogene 1994, 9, 2663–2674; Driscoll, B. et al. Am. J. Physiol. 1997, 273 (Lung Cell. Mol. Physiol.), L941–L949; and Sang, J. et al. Chin. Sci. Bull. 1999, 44, 541–544). There is thus an extensive body of literature validating the use of compounds inhibiting targets in the Cdk4 pathway as anti-proliferative therapeutic agents.
Several small molecules have been identified as Cdk inhibitors and have been the subject of recent reviews (Webster, Exp. Opin. Invest. Drugs, Vol. 7, pp. 865–887 (1988), Stover, et al., Curr. Opin. In Drug Discov. and Devel., vol. 2, pp. 274–285 (1999) and Toogood, Med. Res. Rev., vol. 6, pp 487–498 (2001).
It is thus desirable to identify chemical inhibitors of Cdk4 kinase activity. It is particularly desirable to identify small molecule compounds that may be readily synthesized and are effective in inhibiting Cdk4 or Cdk4/cyclin complexes, for treating one or more types of tumors.
There are several examples of small molecule inhibitors of the cyclin-dependent kinases, including Cdk4 (Rosania, G. R. et al. Exp. Opin. Ther. Patents 2000, 10, 215–230). Several of these compounds inhibit multiple targets.
For example, Flavopiridol (Aventis)
is in Phase II clinical trials for lymphoma and multiple myeloma and also for the treatment of solid tumors. It is said to inhibit Cdk1, Cdk2 and Cdk4 and block cells in both G1 and G2 phases. It is also a weaker inhibitor of PKC and EGFR (Senderowicz, A. M. et al. J. Natl. Cancer Inst 2000, 92, 376–387).
WO9716447 (Mitotix) discloses the following compounds related to flavopiridol

Some of these compounds are stated to inhibit Cdk4.
WO9943675 and WO9943676 (Hoechst) disclose the following purine derivatives
which are stated to inhibit Cdk2 and Cdk4.
WO9833798 (Warner-Lambert) discloses the following pyridopyrimidines

These compounds are stated to inhibit the cyclin dependent kinases Cdk1, Cdk2, and Cdk4. Some of these compounds also inhibit the receptor tyrosine kinases PDGFR and EGFR, and the cellular Src protein kinase, c-Src.
WO9909030 (Warner-Lambert) discloses naphthyridinones
that inhibit PDGFR, FGFR, c-Src, and the cyclin dependent kinases Cdk1, Cdk2, and Cdk4.
WO0039101 (AstraZeneca) discloses diaminopyrimidines
that inhibit Cdk4 and FAK3.
WO0012485 (Zeneca) discloses diaminopyrimidines
that inhibit Cdk4 and FAK3.
WO9924416 (Bristol-Myers Squibb) discloses aminothiazole inhibitors of formula

The compounds inhibit Cdk1, Cdk2 and Cdk4.
WO9921845 (Agouron) discloses diaminothiazole inhibitors of Cdk1, Cdk2 and Cdk4, having the following structure
where R1 and R2 are ring systems. This patent application states that when the R2 ring system does not bear an ortho substituent, the compounds lack potency and selectivity as inhibitors of Cdk4.
WO0075120 (Agouron) discloses diaminothiazole inhibitors of protein kinases including VEGF-R, FGF-R, CDK complexes, TEK, CHK1, LCK, and FAK, having the following structure

WO0026202 (Pharmacia & Upjohn S.p.A., Italy) discloses 2-amino-thiazole derivatives of formula

These compounds are asserted to be antitumor agents operating by a kinase dependent mechanism.
WO1056567 (Novo Nordisk) discloses diaminothiazoles of formula
as GSK-3 inhibitors. These compounds are stated to be useful in treating type 2 diabetes.
WO0160816 (Amgen) discloses pyrimidines of formula

These compounds are asserted to modulate kinase activity.
WO0179198 (Agouron) discloses amino-pyrazoles of formula

These compounds are asserted to mediate/inhibit Cdks, VEGF, and CHK1, and to be useful in treating cancer.
WO 02/12250 A2 (Agouron) discloses pyrazole-thiazole compounds of

These compounds are inserted to be Cdk4/Cdk2 inhibitors.
WO 2004/014904 (Agouron) discloses compounds of formula

The compounds are said to be protein kinase inhibitors, particularly for Cdk2 and Cdk4.
WO 03/097048 (F. Hoffmann-LaRoche AG) discloses compounds of formula:

These compounds are said to inhibit cyclin-dependent kinase 4 (Cdk4) and to be selective against Cdk2 and Cdk1
It is thus desirable to provide small molecule inhibitors of Cdk4 that preferably are selective against other Cdks. That is, the small molecule preferably is selectively more inhibitory of Cdk4 than Cdk1 or Cdk2, most preferably selectively more inhibitory of Cdk4 than Cdk1 and Cdk2. In a preferred embodiment, the compounds of the invention are at least five times more inhibitory of Cdk4 than Cdk1 or Cdk2, most preferably five times more inhibitory of Cdk4 than Cdk1 and Cdk2. Selectivity is desirable because of the potential concomitant toxicity and other undesirable complications that may follow from inhibiting multiple targets. Thus, a compound that exhibits selectivity against Cdk2 and Cdk1 is expected to have a better safety profile than a compound that is not selective between Cdk4, Cdk2 and Cdk1. The present invention provides such compounds, compositions containing such compounds, and methods of using such compounds in the treatment or control of breast, colon, lung, and prostate tumors.